Earlier devices for displacing tissues inside an organism, e.g. bone tissues, the geometry of which may be modified progressively and reversibly without re-operation nor anesthesia, use a threaded rod which works in compression and the loaded length of which increases as the device elongates, with an increasing buckling risk and with the necessity, in order to obtain sufficient stress to rotate the screw, to use a reducer system, which is space eating and expensive, such as those disclosed in documents U.S. Pat. No. 6,849,076, US20070270803 or WO2009058546.